This invention relates to a nuclear reactor fuel element. More specifically this invention relates to a nuclear reactor fuel element having a layer of vanadium as an oxygen getter on the inside surface of the cladding. This invention also relates to a method for coating the inner surface of the cladding with a layer of oxygen gettering material.
A continuing supply of fissionable material is necessary to fuel future nuclear power plants to ensure adequate electrical power to meet the needs of the future. At present, power reactors are fueled with fissionable uranium-235 of which only a limited supply is available. To overcome this shortage of fissionable material, "breeder" power reactors are being developed, which produce more new fissionable material than is consumed in sustaining the reaction. For example, fissionable .sup.233 U or .sup.239 U is bred from fertile .sup.232 Th or .sup.238 U which is relatively abundant.
Because of their desirable physical characteristics, a reactor fuel of mixed plutonium and uranium oxides is being considered to fuel the breeder reactors presently under development. However, several problems have been discovered which are associated with the use of the mixed oxides. For example, it has been found that mixed oxide fuels are far more oxidizing than uranium oxide when used alone as a fuel. This oxidizing power, also known as the oxygen potential, is a measure of the driving force for the numerous reactions which take place in the fuel element during irradiation. Among other problems, this oxidizing potential provides the chemical driving force for corrosive attack of the fuel element cladding, and controls the vapor pressure of many fuel components, especially that of the uranium oxides and thus affects redistribution of uranium in the mixed oxide fuel matrix. The oxidizing potential also controls the chemical state of many fission products, whose interaction with the fuel contributes to fuel swelling, volatility and redistribution.
A particular problem has been the attack by the oxygen upon the fuel element cladding. Two types of cladding attack have been observed at the fuel-cladding interface. One is a general recession of the cladding thickness by a uniform oxidation of the stainless steel, known as matrix attack. The second is intergranular penetration by oxygen and fission products along grain boundaries in the cladding.
The fission products cesium, molybdenum, tellurium and iodine are found in the gap between the cladding and the fuel and are significant factors in influencing the degree and type of cladding attack. Fission-product cesium and molybdenum are found within the grain boundaries of the cladding. Chromium and manganese, which originate in the stainless steel matrix, are found in the fuel-cladding gap along with the fission products.
Cladding attack generally occurs at temperatures above 500.degree. C. and is highly localized. Only rarely does cladding attack proceed in a continuous manner along an appreciable length of cladding. Matrix attack rarely exceeds depth of 2 mils, while grain boundary attack may penetrate the entire thickness of the cladding and appears to be mainly the function of the initial oxygen to metal ratio of the fuel.
A nuclear reactor fuel element which seeks to solve some of these problems is described in U.S. Pat. application Ser. No. 499,958, filed Aug. 23, 1974, and assigned to the common assignee. This fuel element contains a layer of chromium on the inner surface of the cladding as a protectant. Also described therein is a method for providing a coating of chromium on the cladding. However, this fuel element has not proven successful because of the inability to provide a layer of chromium on the surface which was sufficiently free of carbon impurities, since the carbon in the chromium will react with stainless steel cladding.